1. Field of the Invention
The invention generally relates to hysteresis circuits. More particularly, the :nvention relates to high-speed temperature compensated hysteresis circuits which guarantee a predetermined minimum hysteresis window over a wide temperature range so that the circuits are suitable for use in military applications and hostile environments.
2. Description of the Related Art
Hysteresis circuits are well known devices for accurately detecting a change in the value of an input signal voltage. These circuits operate by varying threshold voltage values so that when the input signal value is "low", a high threshold, V.sub.IH, is set and when the input signal is "high" a low threshold value V.sub.IL is set. By keeping the hysteresis window, i.e., the absolute value of the voltage difference of V.sub.IL and V.sub.IH, above some minimum value, reasonable assurance may be had that noise or transients do not inadvertently cause a threshold boundry to be crossed to yield a false signal that a swing in input signal voltage has occured.
These circuits must operate to detect valid threshold crossings and adjust threshold levels accordingly, i.e., if the V.sub.IL threshold is set and crossed the circuit must operate to set the V.sub.IH threshold and vice versa.
Known hysteresis circuits are not output independent. Typically, signals output by a hysteresis circuit are also fed back to the circuit itself to set up the hysteresis reference levels (the threshold voltages V.sub.IL and V.sub.IH). As a result, the hysteresis reference levels may be affected by variables in the output stage, for example, output current. It would therefore be desirable to have a hysteresis circuit which operates in parallel with data being propagated to the output stage, independent of the output itself, to eliminate such variables.
Furthermore, it has been determined that it would be desirable to reduce I.sub.CC, the total current required by a hysteresis circuit, from the I.sub.CC levels required by known circuits in order to meet military application specifications.
Still further, it has been determined that it would be desirable to be able to improve the input characteristics of known hysteresis circuits by minimizing I.sub.IL ("current when input is low") to, for example, achieve improved fan-in.
Further yet, it would be desirable to insure that hysteresis circuits having all the features set forth hereinbefore assure a minimum preselected hysteresis window over wide temperature ranges. In particular, this feature is desirable over the 210.degree. C. operating range between -55.degree. C. and 155.degree. C. typically required for military applications. This assurance that a minimum preselected hysteresis window can be guaranteed over a wide temperature range would allow for the incorporation of these circuits in hostile environments generally. Known hysteresis circuits simply do not have this capability.
Finally, it would be advantageous if hysteresis circuits having all the features set forth hereinbefore, were self-compensating with respect to the components used to fabricate the circuit itself. A self-compensating feature to account for manufacturing disparities in components would further improve the reliability and accuracy of such circuits.